I think that is an advanced use, rather than a common use. I would prefer that
to be something you manually code.
-- Howard.
> On 16 Dec 2017, at 7:08 am, Tony Allevato <tony.allev...@gmail.com> wrote:
>
>
>
>> On Fri, Dec 15, 2017 at 11:39 AM Howard Lovatt via swift-evolution
>> <swift-evolution@swift.org> wrote:
>> +1
>> I think the simple solution of if you provide either == or hashValue you
>> have to provide both is the best approach. Good catch of this bug.
>> -- Howard.
>
> That would be a significant usability hit to a common use case. There are
> times where a value is composed of N fields where N is large-ish, and
> equality is dependent on the values of all N fields but the hash value only
> needs to be "good enough" by considering some subset of those fields (to make
> computing it more efficient).
>
> That still satisfies the related relationship between == and hashValue, but a
> user wanting to explicitly implement a more efficient hashValue should *not*
> necessarily be required to explicitly write the same == that would be
> synthesized for them in that case.
>
>
>>
>> > On 16 Dec 2017, at 6:24 am, Daniel Duan via swift-evolution
>> > <swift-evolution@swift.org> wrote:
>> >
>> > +1. The proposal wasn’t explicit enough to have either supported or be
>> > against this IMO. It’s a sensible thing to spell out.
>> >
>> > Daniel Duan
>> > Sent from my iPhone
>> >
>> >> On Dec 15, 2017, at 9:58 AM, Joe Groff via swift-evolution
>> >> <swift-evolution@swift.org> wrote:
>> >>
>> >> SE-0185 is awesome, and brings the long-awaited ability for the compiler
>> >> to provide a default implementation of `==` and `hashValue` when you
>> >> don't provide one yourself. Doug and I were talking the other day and
>> >> thought of a potential pitfall: what should happen if you provide a
>> >> manual implementation of `==` without also manually writing your own
>> >> `hashValue`? It's highly likely that the default implementation of
>> >> `hashValue` will be inconsistent with `==` and therefore invalid in a
>> >> situation like this:
>> >>
>> >> struct Foo: Hashable {
>> >> // This property is "part of the value"
>> >> var involvedInEquality: Int
>> >> // This property isn't; maybe it's a cache or something like that
>> >> var notInvolvedInEquality: Int
>> >>
>> >> static func ==(a: Foo, b: Foo) -> Bool {
>> >> return a.involvedInEquality == b.involvedInEquality
>> >> }
>> >> }
>> >>
>> >> As currently implemented, the compiler will still give `Foo` the default
>> >> hashValue implementation, which will use both of `Foo`'s properties to
>> >> compute the hash, even though `==` only tests one. This could be
>> >> potentially dangerous. Should we suppress the default hashValue
>> >> derivation when an explicit == implementation is provided?
>> >>
>> >> -Joe
>> >> _______________________________________________
>> >> swift-evolution mailing list
>> >> swift-evolution@swift.org
>> >> https://lists.swift.org/mailman/listinfo/swift-evolution
>> > _______________________________________________
>> > swift-evolution mailing list
>> > swift-evolution@swift.org
>> > https://lists.swift.org/mailman/listinfo/swift-evolution
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution@swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution
